1. Field of the Invention
The present invention relates to a method for crystallizing silicon, and more particularly, to a method for crystallizing silicon using a sequential lateral solidification (SLS) process.
2. Discussion of the Related Art
As modem society has quickly changed into an information-oriented society, flat panel displays are widely used because they have many advantages such as a slim profile, lightweight, low power consumption and the like. Among the flat panel displays, liquid crystal displays (LCD) having superior color reproduction have been developed.
Generally, the LCD includes two substrates facing each other. Electrodes are formed on facing surfaces of the substrates, and a liquid crystal material is injected into a space between the substrates. Therefore, the LCD displays an image by applying a voltage to electrodes to vary the twist of liquid crystal molecules to vary the light transmissibility through the liquid crystal.
A lower substrate of the LCD includes thin film transistors (TFT) and an active layer that is typically formed of amorphous silicon (a-Si:H).
As amorphous silicon can be deposited in a thin film at a relatively low temperature, it is widely used for forming TFTs in a liquid crystal panel with a substrate formed of a glass having a relatively low-melting point.
However, the amorphous silicon thin film has a problem in that it deteriorates the electric characteristics and reliability of the TFTs in the liquid crystal panel, and make it difficult to produce LCDs with large screen sizes.
For a large-sized, high-definition panel image driver circuit, a laptop computer, and a wall-mountable LCD TV, a pixel-driving device having improved electrical characteristics (i.e., a high electric field effect mobility (30 cm2/VS), a high frequency performance characteristic, and a low leakage current) is required. A high quality polycrystalline silicon improves the electrical characteristics of the TFTs.
The electrical characteristics of the polycrystalline silicon thin film particularly depend on the size of the grain, that is, the greater the size of the grain, the greater the electric field effect mobility.
Accordingly, methods for single-crystallizing silicon has become a major issue in the art. PCT Publication No. WO 97/45827 and Korean Unexamined Patent No. 2001-004129 disclose a sequential lateral solidification (SLS) technique for making a massive single crystalline silicon structure by inducing lateral growth of a silicon crystal using a laser as an energy source.
The SLS technique relies upon the fact that a silicon grain grows in a direction normal to a boundary surface between liquid silicon and solid silicon as heated silicon cools. Such an SLS technique crystallizes an amorphous silicon thin film by letting the silicon grain laterally grow to a predetermined length, by appropriately adjusting laser energy intensity and a range between the laser and the silicon.
A method for crystallizing the silicon using the SLS technique will now be described in conjunction with the accompanying drawings.
FIG. 1 shows an SLS apparatus used for a conventional SLS crystallizing method.
An SLS apparatus 100 includes a laser generator 111 for generating a laser beam 112, a convergence lens 113 for converging the laser beam 112 irradiated from the laser generator 111, a mask 114 for dividing the laser beam into a plurality of sections and projecting the divided sections onto a substrate 116, and a scale lens 115 for reducing the laser beam 112 passing through the mask 114 by a predetermined factor.
The laser generator 111 irradiates a laser beam 112, and the irradiated laser beam 112 is adjusted in its intensity while passing through an attenuator (not shown) and then passes through the mask 114 and through the convergence lens 113. Amorphous silicon is deposited on the substrate 116, which is disposed on an X-Y stage 117.
To crystallize the entire area of the substrate 116, a method for gradually enlarging the crystallized area by minutely moving the X-Y stage 117 is used.
The mask 114 is divided into laser beam transmission regions 114a allowing for the transmission of the laser beam 112, and laser beam shielding regions 114b for absorbing the laser beam 112.
The distance between the transmission regions 114a (the width of each shielding region 114) determines the length of a laterally grown grain.
A method for crystallizing the silicon using the above-described SLS apparatus will now be described.
Generally, crystalline silicon is used for forming a buffer (insulating) layer (not shown) on the substrate 116, and amorphous silicon is deposited on the buffer layer.
The amorphous silicon layer is deposited on the substrate 116 using, for example, a chemical vapor deposition (CVD) process, in the course of which a large amount of hydrogen can be retained in the amorphous silicon layer.
Because the hydrogen tends to separate from the thin film by heat, the amorphous silicon layer may undergo dehydrogenization through heat treatment.
When the hydrogen is not removed in advance, the crystallized layer may be exfoliated due to the rapid volume expansion of the hydrogen gas retained in the amorphous silicon layer during the crystallization process.
In addition, the crystallization process using the laser cannot simultaneously crystallize the entire area of the surface. Because the width of the laser beam 112 and the size of the mask 114 are limited, to crystallize the large-sized screen panel, a single mask 114 is moved many times and the crystallization process is repeated after the mask 114 is moved.
A unit block is an area that is crystallized that is as large as the area of the single mask 114, and the crystallization of the unit block may be realized by repeatedly irradiating the laser beam.
FIG. 2 is a graph illustrating the crystallization of silicon with respect to laser energy density, in which laser crystallization regions are classified according to grain size.
As shown in FIG. 2, a first range is the partial melting range. In the partial melting range, only the surface of amorphous silicon layer is molten to form small grains.
A second range is a near complete melting range. In the near complete melting range, grains having a size larger than the size of the grains of the first range can be formed, but it is difficult to form grains having a uniform size.
A third range is a complete melting range. In the complete melting range, the whole amorphous silicon layer is molten and then fine grains are formed due to homogeneous nucleation.
In the conventional method for crystallizing silicon using the SLS technique, the crystallization is performed using a laser energy density in the complete melting range (i.e., third range).
FIG. 3 shows a schematic plan view illustrating a mask used for crystallizing silicon using the conventional SLS technique.
As shown in the drawing, a mask 114 includes transmission and shielding regions 114a and 114b. Each of the transmission regions 114a is made in the form of a long slit extending in a first direction.
A width d of the transmission region 114a is designed to be less than twice as long as a maximum length of a grain grown by a primary laser irradiation process.
When the laser beam is irradiated through the mask structure onto the silicon, the grains grow laterally in molten regions of the amorphous silicon layer, which correspond to the transmission regions of the mask. The grains laterally grow from both boundaries of the molten region until they contact each other at a middle portion of the molten region.
During the crystallization process, the beam pattern passing through the mask 114 and reduced by the scale lens 115 (see FIG. 1) moves in a direction of an X-axis.
The crystallization process proceeds while the laser pattern moves in steps of approximately 200 μm to 100 mm over the length of the mask (i.e., the length of a pattern as reduced by the scale lens 115).
The crystallization method using the conventional SLS technique will now be described in more detail with reference to FIGS. 4A through 4C, which illustrate an example of a 2-shot SLS poly silicon crystallization method. In this example, it is assumed that three transmission patterns (regions) are defined on the mask.
In the 2-shot poly crystalline silicon crystallization method, regions of the amorphous silicon layer that correspond to the transmission regions are crystallized by irradiating the laser beam twice. In addition, this crystallization process is repeatedly carried out in a lengthwise direction. When the crystallization is completed in the lengthwise direction of the substrate, the laser pattern minutely moves in a direction substantially perpendicular to the lengthwise direction and then moves lengthwise to proceed with the crystallization thereby completing the crystallization process for the desired region.
In more detail, the mask 114 (see FIG. 3) is initially located at a starting point on the substrate, and the first laser beam is irradiated to begin the crystallization process for the amorphous silicon layer deposited on the transparent insulating substrate.
The irradiated laser beam is divided into a plurality of sections while passing through the plurality of slits 114a (see FIG. 3) formed on the mask 114. Regions of the amorphous silicon layer, corresponding to the slits 114a are liquefied by the divided laser beams.
In this case, laser energy intensity is set to be in the complete melting range in which the amorphous silicon layer becomes completely molten.
When the laser beam irradiation is completed, silicon grains grow laterally at a boundary between the solid amorphous silicon region and the liquefied amorphous silicon region.
The width of the beam pattern on the substrate is set to be less than twice as long as a length of the grown grain. In addition, the crystallized regions correspond to the transmission regions 114a (see FIG. 2) of the mask. Therefore, each of the crystallized regions A1, A2, and A3 has a length corresponding to that of each of the transmission regions 114a. Regions of the amorphous silicon layer, which correspond to the shielding regions 114b (see FIG. 2) of the mask, remain amorphous silicon regions 167.
In the crystallized regions A1, A2, and A3, the grains 166a and 166b grow laterally from the boundaries between the liquefied silicon and the solid silicon, thereby defining a grain boundary as shown in FIGS. 4A to 4C.
Afterwards, the crystallization process is repeatedly carried out in a direction of the X-axis while the stage carrying the substrate moves by approximately 200 μm to 100 mm over the length of the mask.
As shown in FIG. 4B, when the first crystallization in the direction of the X-axis is completed, the mask 114 or the X-Y stage 117 (see FIG. 2) is minutely moved in a direction of a Y-axis.
Next, a second laser irradiation is initiated from a point where the first crystallization is finished in the direction of the X-axis. Through the second laser irradiation, the grains of the crystallized silicon formed by the first laser irradiation are further consecutively grown. The grains are further grown to have a length that is half as long as a distance “k” from the grain boundary 116c of the crystallized region A1 to the grain boundary of the adjacent crystallized region A2.
Accordingly, as shown in FIG. 4C, a polycrystalline silicon thin film formed of the grains 168a and 168b having a predetermined length can be realized.
At this point, in newly crystallized regions B1 and B2, grains 168a and 168b are vertically grown from the boundaries between the liquefied silicon and the solid silicon. The grains 168a and 168b are further grown until they contact each other, thereby defined a new grain boundary 168c. A part of the crystal formed thus is shown in a partially enlarged view of FIG. 4C.
The related art method for crystallizing amorphous silicon using the aforementioned SLS technique is carried out in the complete melting range where the amorphous silicon is completely melted. The lateral crystallization in the complete melting range may include many inner defects 169 inside large grains. The inner defects 169 are cured by performing a secondary excimer laser annealing (ELA) in the near complete melting range having an energy density less than an energy density of the complete melting range.
However, the related art method using the high energy laser beam twice and the low energy laser beam once increases the overall process time of the crystallization, so that the process yield decreases.